Résumé
The Southwest Indian Ridge (SWIR) in its eastern part is characterized by very low magmatic inputs at ultraslow spreading rates and is therefore a natural laboratory to study oceanic accretion in an extreme thermal configuration. Plate spreading is mainly accommodated by large offset normal faults, also called detachment faults. They exhume mantle peridotites to the seafloor in a nearly amagmatic spreading. These ridge portions are a plausible analog for processes occurring at ridge cold spots as the ridge transform intersections at megatransform domains. Using 99 samples of partially serpentinized peridotites dredged from the eastern termination of the Southwest Indian Ridge, we characterize the deformation processes active in the root zone of the detachment fault system. The deformation is heterogeneous even at the sample scale and combines both brittle and crystal-plastic mechanisms. Strain localization is initially controlled by grain-scale strength contrasts between olivine and orthopyroxene and among variably oriented olivine crystals. Orthopyroxene deformation is primarily brittle (microfractures), but kinks and dynamic recrystallization are locally observed. In contrast, olivine deforms primarily by dislocation creep with dynamic recrystallization under high deviatoric stresses (80-270 MPa) at high temperature (>800 degrees C). Dynamic recrystallization controlled by strain and stress concentrations produce anastomosing zones of grain size reduction (GSR). This heterogeneous high-stress deformation is observed, with variable intensity, in every sample investigated, suggesting that it was pervasively distributed in the root region of axial detachments. We use these microstructural observations to constrain a 2D thermomechanical model of lithospheric extension, in which we explore two weakening mechanisms as seen in the samples: serpentinization (T< 350 degrees C) and grain size reduction (T> 800 degrees C). The combination of the two-deformation mechanisms leads to the nucleation of detachment faults in a thick lithosphere evolving to a flipflop pattern similar to that observed at the eastern SWIR (Bickert et al., 2020).